posixtm: remove PDS_LEADING_YEAR This changes the API slightly, in a hopefully-innocuous way. Without this change the code had undefined behavior when a caller specified neither PDS_LEADING_YEAR nor PDS_TRAILING_YEAR. Problem reported by Pádraig Brady in: https://lists.gnu.org/r/bug-gnulib/2017-11/msg00048.html * NEWS: Mention this. * lib/posixtm.c (posix_time_parse): Treat the absence of PDS_TRAILING_YEAR as if PDS_LEADING_YEAR were present. * lib/posixtm.h (PDS_LEADING_YEAR): Remove (actually, leave it present, but define it as zero, for compatibility with existing source code). All other PDS_* values moved up. * tests/test-posixtm.c (LY): New macro. (T): Use it. Do not expect a particular numeric encoding for PDS_CENTURY etc.